The fatigue strength of weld surfaces on truck frames has been estimated according to the fatigue limit diagram in JIS E 4207 - General Rules for Design of Truck Frames for Railway Rolling Stock. However, it is not possible to predict actual fatigue life using this diagram. This study demonstrates the possibility of fatigue life estimation of welds on truck frames based on a modified Miner's rule. In order to estimate fatigue life, three factors are needed: the S-N diagram prepared based on the allowable stress in JIS E 4207 or the JSSC fatigue design curve, the relationship between crack occurrence and running distance, and the frequency distribution of stress caused by service loads.